Known in the past has been an internal combustion engine providing an air-fuel ratio sensor in an exhaust passage of the internal combustion engine and controlling an amount of fuel which is fed to the internal combustion engine based on an output of the air-fuel ratio sensor.
The air-fuel ratio sensor used in such an internal combustion engine gradually deteriorates along with use. As such deterioration, for example, deterioration of response of the air-fuel ratio sensor may be mentioned. The deterioration of response of the air-fuel ratio sensor occurs due to air holes provided in a sensor cover for preventing a sensor element from being covered by water ending up being partially clogged by particulate matter (PM). If the air holes are partially clogged in this way, the exchange of gas between the inside and outside of the sensor cover becomes slower, and as a result the output of the air-fuel ratio sensor ends up becoming blunter. If such deterioration of the air-fuel ratio sensor occurs, the various control operations performed by the control system of an internal combustion engine end up being hindered.
Therefore, diagnosis systems diagnosing the abnormality of deterioration of response of an air-fuel ratio sensor have been proposed (for example, see PLTs 1 to 4). In such diagnosis systems, as a diagnosis system diagnosing abnormality of an air-fuel ratio sensor arranged at a downstream side from an exhaust purification catalyst (below, referred to as the “downstream side air-fuel ratio sensor”), for example, a system diagnosing abnormality of the downstream side air-fuel ratio sensor during fuel cut control stopping or greatly reducing the feed of fuel to a combustion chamber has been proposed (for example, PLT 1).
In particular, in the diagnosis system described in PLT 1, during fuel cut control, the time period of passage when an air-fuel ratio corresponding to the output of the downstream side air-fuel ratio sensor (below, referred to as the “output air-fuel ratio”) passes through a predetermined air-fuel ratio region is used as the basis for diagnosis of abnormality. Specifically, when the time period of passage is larger than a predetermined abnormality judgment value, the downstream side air-fuel ratio sensor is diagnosed as suffering from the abnormality of deterioration of response. In addition, in the diagnosis system described in PLT 1, considering the trends in the exhaust air-fuel ratio right before start of fuel cut control and the changes in the time period of passage depending on the oxygen storage amount of the exhaust purification catalyst, if the integrated value of the intake air in the period from the start of fuel cut control to when the output air-fuel ratio reaches a predetermined air-fuel ratio richer than the predetermined air-fuel ratio region is less than a predetermined threshold value, the downstream side air-fuel ratio sensor is not diagnosed for abnormality.